1. Field of the Invention
This invention relates to frequency converters and, more specifically, to static electromagnetic subharmonic frequency changers. It is specifically concerned with the generation of subharmonics in a frequency changer using a saturable reactor as a modulation device.
2. Prior Art
Static electromagnetic frequency changers, particularly those which divide frequencies, operate by generating subharmonic oscillations in a circuit including inductive and capacitive reactances. The major component of a static electromagnetic frequency changer is a nonlinear or saturable reactor which functions in a modulating mode. This saturable reactor is simultaneously energized by a low frequency signal and a high frequency signal which are related to each other by a predetermined functional relation. Normally the higher frequency signal is supplied by a sinusoidal driving source. The lower frequency signal is produced by a resonant circuit which is excited by the driving source. The saturable reactor acts as a mixer or modulator and produces a plurality of summation frequencies and sideband frequencies. It has been determined that the lower sideband frequency in such a system introduces negative impedance into the path of the lower frequency signal. The signal due to this lower sideband frequency flows through the saturable reactor and causes energy to be abstracted from the higher frequency source signal and delivered to the lower frequency signal generated by the excited lower frequency resonant circuit.
In order to generate desired subharmonics, a definite relation must exist between the signal frequencies. This relation is expressed by the equation: EQU f.sub.3 = mf.sub.1 .+-. nf.sub.2 ( 1)
where:
f.sub.1 is a signal frequency generated by the driving source; PA1 f.sub.2 is a signal frequency generated by a resonance in the circuit; PA1 f.sub.3 is a subharmonic resulting from a nonlinearity in the circuit; and PA1 m and n are integers.
In the equation the signal frequencies, or signals at frequency f.sub.3, flow in a negative impedance path. If f.sub.1 and f.sub.2 are commensurable, subharmonics f.sub.3 of the driving source signal are generated.
A typical practical circuit to generate such subharmonics comprises a plurality of resonant circuit paths. Generally, one circuit path is resonant to the frequency of the driving source and another circuit path is resonant to a subharmonic thereof, generally a sideband frequency. These resonant circuit paths include inductive and capacitive reactances and the signals excite a ferroresonant or saturable reactor in common with the two circuit paths.
A typical subharmonic frequency generator is disclosed in U.S. Pat. No. 3,387,203, issued June 4, 1968, to J. J. Munnelly, and assigned to the same assignee as this application. The subharmonic generator disclosed therein comprises a driving source of 60 Hz, a first resonant or idler circuit including a saturable reactor and tuned to 100 Hz, and a second series resonant circuit tuned to 20 Hz. The various frequencies generated conform to the above-described equation (1) and at the output of the transformer a subharmonic signal of 20 Hz is provided. This circuit has two distinct magnetic components, a saturable reactor included in the idler circuit, and an inductance in the series resonant circuit which is included as part of the output transformer. The typical subharmonic type frequency divider of the prior art has required a plurality of magnetic components which tend to make the generator physically heavy and generally substantial in bulk.
An operating characteristic of prior art subharmonic frequency generators is their sensitivity to the amplitude and frequency of the driving source signal. It is a general requirement that the driving source signals must be within a definite, prescribed range of amplitude and frequency in order to maintain the precise frequency output of the generator. If the amplitude of the driving signals go beyond a certain limited range, the oscillations in the resonant circuits of the subharmonic generator fall to zero and the circuit ceases to operate. A small deviation in the frequency of the driving signal significantly changes the frequency of the output of this subharmonic generator. This is a distinct disadvantage particularly in circuits which operate in response to commercial alternating current power where substantial drops in voltage amplitude or brownouts occur.
It is also a serious limitation in subharmonic frequency generators driven by reserve power systems where the frequency may vary by as much as 10 percent.